Recursion trick in ShaderEffectSource fails when I destroy the source object in onStateChanged

Stefan Monov76

I want a ShaderEffectSource called snapshotter that updates on every change of a source item, but when the source item is destroyed, I want snapshotter to retain the last state of the source item.

In my code I use a workaround for a Qt deficiency - setting snapshotter.sourceItem = snapshotter when the previous source item gets destroyed. This works fine when I destroy the old source item e.g. on key press. But when I do it in an onStateChanged handler, I get this error:

ShaderEffectSource: 'recursive' must be set to true when rendering recursively.

But I don't want to set recursive: true, because then snapshotter would start repainting as fast as possible, wasting a lot of processing power.

Any idea why that problem happens considering it works fine when using key press, and/or a workaround?

I realize I'm asking why a hacky solution fails in some cases, but since it works in some cases, I'd like to use the solution anyway as it's very useful.

My code: (main.qml)

import QtQuick 2.6
import QtQuick.Window 2.2

Window {
    visible: true
    width: 640
    height: 480

    Loader {
        active: true
        id: loader
        sourceComponent:
            Rectangle {
                color: "red"
                border.color: "white"
                width: 100
                height: 100
                parent: row

                states: [
                    State {
                        // the condition is always true, in this example code
                        name: "theOnlyState"; when: width === 100
                    }
                ]
                onStateChanged: {
                    if(state === "theOnlyState") {
                        loader.active = false;
                    }
                }
            }
    }

    Row {
        id: row
        ShaderEffectSource {
            id: snapshotItem
            sourceItem: {
                if(loader.status === Loader.Ready) {
                    return loader.item;
                } else {
                    return snapshotItem;
                }
            }
            live: loader.status === Loader.Ready
            width: 100
            height: 100
        }
    }
}

Note: I just had an idea: maybe setting recursive: true will not create the problem I mentioned, considering I'd only set it when live == false. Maybe Qt is smart enough not constantly redraw in that case. But I'm not sure how to check if that's true.

Stefan Monov76

Found a solution and posted it at StackOverflow.

? Offline

Thank you for sharing your solution :-)